This invention concerns a turbine disc. It also concerns a turbine wheel comprising the disc, and a turbine engine or machine comprising the turbine wheel.
With reference to the accompanying drawings a prior art turbine disc is illustrated in FIGS. 1A and 1B in which:
FIG. 1A is a fragmentary and diagrammatic view, partly in section, of the periphery of a known turbine disc; and
FIG. 1B is a section on line IBxe2x80x94IB in FIG. 1A.
With reference to FIGS. 1A and 1B a known turbine disc is shown at A rotatable about an axis and having a peripheral face B extending circumferentially about the axis. A root slot C to engage roots of turbine blades to be held in the turbine disc is formed in the peripheral face B. The root slot C extends circumferentially around the turbine disc and as shown in FIG. 1B has a cross-sectional shape which is adapted to receive and securely hold the blade roots. It will be noticed that root slot C has a re-entrant or undercut shape at the deepest and widest part of its cross-section having opposed concave recesses E1 and E2, which are spaced more widely apart than the convex ribs F1, F2 immediately above. Consequently, the root slot C includes in its side walls cut-away portions J1, J2 comprising opposite arcuate bights in the convex ribs F1, F2. These bights together form a load slot J1, J2 for allowing entry and exit, or xe2x80x9cloadingxe2x80x9d and xe2x80x9cunloadingxe2x80x9d, of the blade roots into the root slot C of the turbine disc A. To attenuate the in-service hoop stress concentrations experienced in the perimeter of the turbine disc as a result of the presence of the load slot J1, J2, F, the root slot C also includes so-called xe2x80x9cshadow slotsxe2x80x9d G1 and G2. Conveniently, each shadow slot G1, G2 comprises opposite arcuate bights each having a radius of curvature which is considerably less than that of the bights J1, J2 forming load slot. However, each bight of a shadow slot G1, G2 extends, through the convex ribs F1, F2 towards the respective concave recess E1 or E2, similarly to the bights J1, J2 forming the load slot.
Turbine discs are known in which the bights constituting the shadow slots have identical radii of curvature, that radius being less than the radius of curvature of the bights constituting the load slot. Furthermore, the bights constituting the shadow slots cause substantial widening of the root slot in the convex ribs above the undercuts.
According to the invention there is provided a turbine disc rotatable about an axis, the turbine disc comprising a peripheral face extending circumferentially about the axis, the peripheral face having a root slot formed therein for engaging roots of turbine blades to be held in the turbine disc, the root slot having a longitudinal extent extending circumferentially around the turbine disc, a depth extending radially of the disc, and a width defined between axially opposed side walls of the root slot, the root slot further having an undercut portion of its cross-section for accommodating the blade roots, at least one of the side walls of the root slot above the undercut portion having cut-away portions including
a) at least one arcuate bight defining a load slot for allowing entry and exit of the blade roots into the undercut portion of the root slot, and
b) on circumferentially opposed sides of the load slot, a further at least one arcuate bight defining a shadow slot for reducing hoop stress concentrations in the perimeter of the turbine disc,
wherein at least one of the bights constituting the shadow slot or slots has a radius of curvature greater than the radius of curvature of the bight or bights constituting the load slot.
It is possible to construct a turbine disc in accordance with the invention above so that when the disc is in use in combination with turbine blades the following advantages may be derived over known turbine discs with conventional shadow slot formation:
i) hoop stress concentration in the vicinity of the load slot (and shadow slots) can be reduced, thus increasing the life of the turbine disc;
ii) crush stress of the turbine blades in the vicinity of the shadow slots is reduced (minimized) by means of increased contact between blade roots and flanks of the ribs at undersides of the ribs; and
iii) the chance of detrimental blade vibration is reduced or avoided because the increased contact mentioned at (ii) above means the natural frequency of the turbine blades is not significantly affected.
The load slot may be constituted by a pair of opposite arcuate bights each in an opposed said side wall.
Each or a said shadow slot may be constituted by a pair of opposite arcuate bights each in an opposed said side wall.
At least one of the bights constituting the shadow slots may have a radius of curvature at least twice as great as the radius of curvature of the bight or at least one of the bights constituting the load slot. For example, at least one of the bights constituting the shadow slots may have a radius of curvature at least substantially eight times greater than the radius of curvature of the bight or at least one of the bights constituting the load slot.
The bights constituting a said shadow slot may have substantially the same radius of curvature.
The side walls of the root slot may comprise axially opposed, circumferentially extending first and second ribs having side faces facing across the root slot, wherein a first maximum distance which said bight or one of the bights constituting the load slot extends axially of the disc into the respective first or second rib from a locus of the side face of the rib is greater than a second maximum distance which said bight or one of the bights constituting the shadow slots extends axially of the disc into the respective first or second rib from the locus of the side face of the rib. Preferably the first maximum distance is at least a plurality of times greater than the second distance. The first maximum distance may be at least substantially twice the second maximum distance. For example, the first maximum distance may be between substantially 2.0 and substantially 2.5 times greater than the second maximum distance.
With respect to at least one of the shadow slots an end of said bight or at least one of the bights constituting the load slot may be circumferentially spaced from an adjacent end of said bight or at least one of the bights constituting the shadow slot.
Alternatively, with respect to at least one of the shadow slots an end of said bight or at least one of the bights constituting the load slot may substantially coincide with an end of said bight or at least one of the bights constituting the shadow slot.
Said ends may be substantially on a said locus.
At least one of the shadow slots may be adjacent to the load slot.
A turbine wheel may comprise a turbine disc formed according to the invention and turbine blades. This turbine wheel may be included in a turbine engine or machine, for example in a gas turbine.
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.